Time resolved optical spectroscopy of the eclipsing intermediate polar EX Hydrae

Time resolved spectroscopy of the eclipsing intermediate polar EX Hya has been obtained. We confirm that the dominant variation in the spectral line flux occurs with the 67-min white dwarf rotation period and is in phase with the 67-min optical and X-ray continuum variation. Further, we find that the pulse fraction is greatest in the wings of the emission lines and that there is a variation in the relative strengths of the blue and red line wings on the 67-min period. The latter is phased so that the blue wing is strongest at maximum flux in the 67-min cycle. We find no evidence for previously reported line profile changes at half the 67-min period. No evidence is found for an occupation of the line flux corresponding to the narrow continuum eclipse that recurs every 98-min orbital cycle. We do, however, find changes in the line profiles that extend for ∼0.15 of the orbital cycle, either side of the photometric eclipse. These we interpret as a progressive occupation of a rotating accretion disc by the companion star. The implied size of the disc is consistent with that inferred from the prominent ‘S-wave’ emission component and from the separation of the emission peaks in the line profiles. A radial velocity semi-amplitude of $$K_1=36\pm9\enspace \text{km} \enspace\text{s}^{-1}$$ is measured, yielding an estimate of the white dwarf mass $$M_1=0.78\pm0.17\enspace M_\odot$$, with the secondary mass $$M_2=0.13\enspace M_\odot$$. The emission line data are interpreted in terms of three components: double peaked emission from a disc, an S-wavc component caused by a bright region on the outside of the disc, and emission pulsed with the 67-min period which originates close to the white dwarf. The pulsed emission probably comes from material flowing down magnetic field lines on to the magnetic pole. Because the field is inclined to the disc, this accretion is asymmetric and occurs predominantly from the side of the disc closest to the magnetic pole. We suggest that the modulation of the line flux is caused by the changing optical depth through the accretion stream as the line of sight rotates. Maximum light occurs when the pole is pointed away from the observer.